Problem: Which of these numbers is prime? ${2,\ 25,\ 55,\ 60,\ 93}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 2 are 1 and 2. The factors of 25 are 1, 5, and 25. The factors of 55 are 1, 5, 11, and 55. The factors of 60 are 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60. The factors of 93 are 1, 3, 31, and 93. Thus, 2 is a prime number.